It is often necessary or useful to know the mass of air flowing through a passageway. While there are many applications for an anemometer or air mass sensor, an application of particular interest is in an internal-combustion engine. For an automobile engine with electronic fuel injection and ignition systems, air mass flow into the engine is one of several important sensed conditions useful in generating an electrical signal which controls and optimizes performance of those systems.
One type of conventional air mass sensor utilizes a temperature-dependent resistive wire, such as platinum wire, having an electrical resistance proportional to its temperature. The resistive wire is placed in the air mass flow of a passageway, and an electrical circuit supplies electric current to the wire. The circuit automatically regulates the flow of current through the resistive wire to maintain its resistance and hence its temperature constant, and measures that current. The measured current (or a voltage proportional thereto) indicates the quantity of air per unit of time flowing through the passageway, and is used by the circuit to generate an air mass flow-indicating signal.
While having proven advantages, such conventional hot wire air mass sensors are frequently expensive, and when used in a harsh environment such as exists in an air induction system of an internal-combustion engine, are inaccurate, unreliable and subject to resistive wire breakage. If the air mass sensor utilizes a bent resistive wire, as have many sensors in the past, the wire is prone to breakage at each bend when heated during sensor operation. Typically, the resistive wire of an air mass sensor operates at temperatures around 250.degree. C., and at such an extremely elevated temperature, resistive wire such as platinum wire becomes brittle in the area of the bend and easily breaks when exposed to vibration and airflow forces.
Even a straight length of heated resistive wire will frequently break when subjected to the vibration and airflow forces encountered in an air induction system of an internal-combustion engine. To minimize the airflow force on the wire, in the past, the wire has been positioned adjacent the passageway walls or in a by-pass duct. Sampling from the air mass flow adjacent to the walls of the air passageway, however, is not representative of the true air mass flow through the passageway due to the drag and turbulence occurring along the walls. Sampling in a by-pass channel is unsatisfactory for similar reasons. Furthermore, when a very low air mass flow is to be sampled, the flow along the walls or in a by-pass duct may not be sufficient to provide an accurate measurement. Hot wire air mass sensors require a minimum air mass flow to operate with an acceptable degree of accuracy.
While it is desirable to sample the air mass flow in the central portion of a passageway, it has proven difficult to do so with a hot wire air mass sensor. If the resistive wire is placed directly in the central portion of the passageway, it is exposed to the airflow having the greatest velocity and exerting the greatest force on the wire. For a typical automobile engine, an air mass sensor must be able to measure air mass flows ranging from 30 to 1,350 pounds per hour. Additionally, the airflow in the passageway in an automobile induction system is far from laminar and unidirectional. As a result of back pressures created by exhaust valves opening and closing, the airflow in the passageway may momentarily reverse direction and apply a significant reverse direction force on the resistive wire.
Another consideration becomes apparent upon a cross-sectional analysis of the airflow in the passageway which indicates that the speed and direction of the airflow varies across the diameter of the passageway and, as discussed above, changes with time. A boundary layer of air along the passageway walls generally flows slower and at any instant of time may be flowing in a direction opposite from that of an adjacent intermediate layer of air which surrounds the airflow in the central portion of the passageway. This intermediate layer, at any instant, may be flowing in a direction and speed different from that of the central airflow. In a conventional automobile engine, this phenomenon is often accompanied by the occurrence of a resonating airflow in the air induction passageway. Consequently, the airflow may apply oppositely directed forces of various magnitudes along the length of a resistor wire if it extends across more than one airflow layer and those forces may periodically change in direction. A resistive wire subjected to such a resonating airflow will itself resonate in response thereto, and the resulting movement of the wire may be sufficient to eventually break it, especially when coupled with the mechanical vibration usually present in an automobile engine.
It will therefore be appreciated that there has been a significant need for a hot wire air mass sensor capable of measuring the air mass flow in the central portion of a passageway and constructed to withstand the vibration and airflow forces typically encountered in an air induction system of an internal-combustion engine. The air mass sensor should be inexpensive, operate at elevated temperatures, provide accurate measurements at low and high air mass flows, and be reliable. The present invention fulfills this need and, further, provides other related advantages.